Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. It has been known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO's) and the aluminophosphates (ALPO's). SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO is generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formulation into a SAPO catalyst, and its use in converting a hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, is shown in U.S. Pat. Nos. 4,499,327; 4,677,242; 4,677,243; 4,873,390; 5,095,163; 5,714,662 and 6,166,282, all of which are herein fully incorporated by reference.
Metalloaluminophosphate molecular sieves contain a pore system, which is a network of uniform pores and empty cavities. These pores and cavities catch molecules that have a size equal to or less than the size of the pores and cavities, and repel molecules of a larger size.
The pores and cavities of molecular sieves are formed as a result of adding template materials during the molecular sieve manufacturing process. During the formation of the molecular sieves themselves, a lattice type chemical structure forms around the template material, with the template material acting as a means of forming the pore structure within the molecular sieve. The resulting molecular sieve may be combined with other components for the benefit of adjusting various properties of the molecular sieve or to form larger particles.
To make the molecular sieve suitable for use, the template must be at least partially, preferably completely, removed so that the pores and cavities are open to catch molecules, either for the purpose of adsorbing the molecules from the environment or to react the molecules to form a desired product. The reaction occurs when the molecules come into contact with catalytic sites located within the pore system, particularly within one or more of the empty cavities or cages as sometimes called.
The template is conventionally removed from the molecular sieve by calcining or burning out the template. An elution process can also be used to remove the template, although calcination is preferred. Once the template is removed, the molecular sieve is considered to be activated or ready for use. The activated molecular sieve has its pore system, including the empty cavities or cages open to the immediate environment, and ready for use.
Activated metalloaluminophosphate molecular sieves that have catalytic sites within their microporous structure, e.g., silicoaluminophosphate (SAPO) molecular sieves, have been found to be sensitive to moisture. In general, significant exposure of the activated molecular sieves to moisture has been found to deactivate the catalytic activity of the molecular sieves. Unfortunately, methods of protecting activated metalloaluminophosphate molecular sieves against the harmful effects of moisture are limited.
U.S. Pat. No. 6,316,683 (Janssen et al.) discloses a method of protecting catalytic activity of a SAPO molecular sieve by shielding the internal active sites of the molecular sieve from contact with moisture. The template itself can serve as the shield, or an anhydrous blanket can serve as a shield for an activated sieve that does not include template. The anhydrous blanket can be a liquid or a gas under standard temperature and pressure conditions, and does not react to any significant degree with the molecular sieve structure under ambient conditions. In the case of an anhydrous liquid blanket, the liquid contains less than about 200 ppm water, preferably less than about 100 ppm water, more preferably less than about 50 ppm water. The anhydrous liquid is preferably selected from the group consisting of alkanes, cyclo-alkanes, C6-C30 aromatics, alcohols, particularly C4+ branched alcohols. It is desirable to shield the active sites, because activated SAPO molecular sieves will exhibit a loss of catalytic activity when exposed to moisture.
U.S. Pat. No. 4,764,269 (Edwards et al.) discloses a method of protecting SAPO-37 catalyst from deactivating as a result of contact with moisture. The catalyst is maintained under storage conditions such that the organic template component of the molecular sieve is retained in the SAPO-37 molecular sieve, until such time as the catalyst is placed into a catalytic cracking unit. When the catalyst is exposed to the FCC reaction conditions, wherein the reactor is operated at 400° C. to 600° C. and the regenerator operated at about 600° C. to 850° C., the organic template is removed from the molecular sieve pore structure, and the catalyst becomes activated for the cracking of hydrocarbons. According to this procedure, there is little if any contact with moisture.
U.S. Pat. No. 6,395,674 to Fung et. al. discloses a method for addressing the problems relating to protecting molecular sieves from damage due to contact with moisture and damage due to physical contact. The method requires the heat treatment of a molecular sieve containing a template under conditions effective to remove a portion of the template from the microporous structure and cooling the heated molecular sieve to leave an amount of template or degradation product thereof effective to cover catalytic sites within the microporous structure.
U.S. Pat. Nos. 6,225,254; 6,448,460; 6,503,863 to Janssen et. al. disclose a method for preserving the catalytic activity of silicoaluminophosphate molecular sieves which comprises heating template-containing silicoaluminophosphate in an oxygen depleted environment under conditions effective to provide an integrated catalyst life which is greater than that obtained using a non-oxygen depleted environment.
U.S. Pat. No. 6,498,120 (Janssen et al.) discloses a method of rejuvenating a molecular sieve with anhydrous liquid or vapor, until the methanol uptake index is increased by at least 10%. In the examples, this document discloses rejuvenation of a SAPO molecular sieve using liquid methanol in admixture with less than 30 wt % water.
U.S. patent application Ser. No. 10/295,994, published 7 Aug. 2003 under No. 2003/0149321 (Mees et al.) and Mees et al., “Improvement of the Hydrothermal Stability of SAPO-34,” Chem. Commun., 2003, (1), 44-45, first published electronically on the web Nov. 22, 2002, discloses a method of protecting SAPO-34 molecular sieve, based on a reversible reaction of NH3 with acid sites of the sieve. The method transforms a H+-SAPO-34 into an NH4+-SAPO-34 in a reversible way. Once they have undergone this treatment, activated metalloaluminophosphate molecular sieves are protected against degradation by water or steam.
U.S. application Ser. No. 10/113,678, published 2 Jan. 2003 under No. 2003/0004056 (Mees et al.), relates to a method of treating at least one activated metalloaluminophosphate molecular sieve with one or more nitrogen containing compounds selected from the group consisting of amines, monocyclic heterocyclic compounds, organonitrile compounds and mixtures thereof under conditions to chemisorb and/or physisorb the nitrogen-containing compound with the metalloaluminophosphate molecular sieve. Once they have undergone this treatment, activated metalloaluminophosphate molecular sieves are protected against degradation by water or steam.
As seen from the disclosures described herein, many metalloaluminophosphate molecular sieves will exhibit a shortened catalytic life when exposed to a moisture-containing environment. This loss of catalytic life, which is due to a loss in the number of acid catalytic sites, can occur over a very short period of time. This loss of catalytic life is sometimes reversible, but if loss of catalytic activity is too severe, it cannot be restored. In addition there may be irreversible loss of molecular sieve crystallinity and porosity on aging during storage and handling after manufacture.
As new large scale, commercial production facilities, which use metalloaluminophosphate molecular sieves in the production process, continue to be implemented, protecting the activated metalloaluminophosphate molecular sieves from loss of catalytic activity as a result of contact with moisture continues to become an even greater challenge. The present invention provides a new method of protecting the catalytic sites of activated metalloaluminophosphate molecular sieves or catalysts containing metalloaluminophosphate molecular sieves during molecular sieve storage and/or handling or during catalyst storage and/or handling.